Carbodiimides may be prepared from isocyanates by a particularly simple reaction even at room temperature based on the process disclosed in German Pat. No. 1,130,594 in which phospholine oxides are used as catalysts. Industrially the most important and effective catalysts which effect very rapid carbodiimidization of virtually any aromatic monoisocyanate and polyisocyanate, even at room temperature, and are capable of converting less reactive aliphatic or cycloaliphatic monoisocyanates and polyisocyanates into carbodiimides at temperatures starting from about 150.degree. C. are those corresponding to the following general formulae: ##STR1##
These catalysts have been used commercially for the production of polycarbodiimide foams.
Experience has shown that carbodiimide formation with the aid of the above-mentioned readily soluble catalysts, which formation proceeds in the form of a homogeneous catalysis, cannot be stopped in such a way that carbodiimides or polycarbodiimides which contain isocyanate groups are obtained as reaction products which are stable in storage. It is likewise not possible to prepare stable solutions of diisocyanatocarbodiimides, such as ##STR2## or .alpha.,.omega.-diisocyanato-bis-carbodiimides, .alpha.,.omega.-diisocyanato-tris-carbodiimides or isocyanato-uretone imines, such as those corresponding to the formula: in excess monomeric monoisocyanates or polyisocyanates. The reason for this failure in the art is that the catalytically highly active soluble phospholine oxides cannot be completely blocked by any of the known inactivating agents, such as phosphorus oxychloride, zinc chloride, dimethyl carbamic acid chloride, benzoyl chloride, hydrochloric acid, boron trifluoride, alkylating agents and the like. Neither the formation of carbodiimides nor the polymerization to polycarbodiimides and crosslinked products can therefore be completely stopped. Thus, because of the progressive (even though in some cases, slow) formation of carbodiimide, a high carbon dioxide pressure soon develops in closed reaction and/or storage vessels, which may result in serious accidents.